Regenerative flow pumps are types of dynamic pumps with the ability
to develop high heads at low flow rates. So they have very low specific
speed and can replace the positive displacement pumps without any wear
and lubrication problems. Furthermore, RFPs are capable of working in
low NPSH. Therefore, Regenerative Pumps are better choice than other
types of centrifugal pumps at low specific speeds [1]. Regenerative
pumps capable of generating heads equivalent to that of several
centrifugal stages with comparable tip speeds. RFPs have compact design
and they are very cheap to manufacture. They present performance curve
with very stable features [2]. Despite the disadvantage of having low
hydraulic efficiency (30-50%), regenerative pumps have found many
applications in industries including automotive and aerospace fuel
pumping, booster systems, water supply, agricultural industries,
shipping and mining, chemical, and food processing systems [3].

Regenerative pumps use a free rotating type impeller, like other
types of turbo- pumps. Impeller has blades machined into each side at
its periphery. As seen in Fig. 1, the fluid moves spirally in flow
channel and reenters the blades of impeller several times in its
peripheral path from inlet to outlet which forms a torroidal motion. The
Fluid enters the flow channel with inlet port and discharges with high
pressure from the outlet port. The stripper occupies the space between
inlet and outlet regions of casing to separate low-pressure inlet from
high-pressure outlet. The stripper helps the fluid to go out from
discharge port. Just the fluid within impeller blades can pass through
stripper.

The idea of regenerative flow pump was presented by Adolph Wahle
and the first model of it was manufactured by Western Pump company at
1918 [5]. Despite existence of several experimental and theoretical
analyzes about regenerative flow pump, the number of articles in this
field is low compared to other types of turbomachines.

Bartles attempted to study the flow mechanism in regenerative pumps
experimentally using three different rotors with the same flow channel.
He found that the pump would only work with the impeller which was
grooved out and allowing circulatory motion and centrifugal pumping [4].
Lazo and Hopkins setup an experimental test rig for a regenerative pump.
They used small thread probes to measure and observe flow velocities and
angles in flow passage of the pump to obtain better information for flow
pattern recognition; at the same time Lutz investigated the pressure
distribution from inlet to outlet of the regenerative pump [3]. Crewdson
studied the circulatory flow between blades and flow channel of
regenerative pump. He divided the flow channel in to two parts by
soldering a thin brass strip along the middle of it. So the circulatory
flow was affected. He concluded that cluttering the circulatory flow
pattern would reduce the head and efficiency of the pump [4]. The effect
of non-radial blades on regenerative pump performance was investigated
by Burton. He found that using a 47[degrees] blade angle would increase
the head about twice of that obtained by radial blades at shut-off point
[6]. A new model for regenerative pump was presented by Badami, in which
both inlet and outlet ports were designed axially. He also studied some
geometric parameters of regenerative pumps and used the momentum
exchange theory proposed by Wilson et al. to present a comparison
between experimental and theoretical data [7]. Engeda reviewed the
status of regenerative pumps and proposed a Modified model which could
describe the change in rotation due to variation in channel region.
Predicted results of the model were compared with experimental data [8].
Yoo et al. studied the design of a regenerative flow pump for artificial
heart pump application based on an improved momentum exchange theory
which could calculate the geometry of rotating flows [9, 10]. Quil et
al. used a commercial code to investigate the fluid flow in a
regenerative pump. They also carried out a new method of manufacturing
to evaluate the influence of change in blade geometry on the efficiency
of regenerative pump [11, 12]. An experimental study was accomplished by
Choi et al. to investigate the effect of the impeller blade angle and
its shape on regenerative pump performance. Among all blade
configurations tested in this study, the chevron blade exhibited the
highest head with reasonably good efficiency. It was found that there
was an optimum chevron angle of around 30[degrees] [13]. Fleder and
Bohle studied the effect of the variation of the blade length, blade
width, and side channel height by the help of dimensionless parameters
[14]. Karanth et al. used CFD analysis to enhance the performance of
regenerative pump. They found that the number of blades and the inlet
and outlet passage of the regenerative pump have significant effect on
the pump performance [15]. Maity et al. also used CFD to study
regenerative pumps. They found that a curvature in the outlet flow
domain minimizes the vortex flow, so the net pressure head increases.
Also, positioning of the blades on either side of the impeller by
offsetting enhances the fluid motion and results into the net increase
in static and net pressure head [16]. Karlsen and Aggidis offered an
extensive review into the development, performance challenges and design
improvements of Regenerative pumps with a particular focus on improving
efficiency throughout the pump life cycle [4]. Nejadrajabali et al.
carried out a numerical study to investigate the effect of blade profile
on the performance of regenerative pump [17].

The above literature indicates that the effect of fluid viscosity
on the performance of regenerative pumps has not been the focus of
studies and hence an attempt has been made in this paper to investigate
experimentally the influence of fluid viscosity on the performance of
regenerative pump. The primary fluid was water with viscosity of 1
centipoise. Polyacrylamide (PAM) which is a soluble polymer in water was
employed to increase the viscosity of fluid. The mixture of water and
PAM was pumped several times for breaking or shearing its molecules at
constant temperature to have a homogenous fluid. A Brookfield viscometer
was used for measuring viscosity. Solutions prepared from dry powder PAM
at two concentrations of 1600 ppm and 2400 ppm. The measured viscosities
for these two solutions were 180 and 300 centipoises respectively. The
performance curves of the regenerative pump working with these fluids
were compared to assess the effect of fluid viscosity on the operation
of regenerative pump.

2. Specifics of regenerative pump

Fig. 2 shows the components of the regenerative pump consists of an
impeller with radial blades at its periphery, inlet and discharge port,
stripper to isolate the high-pressure discharge from the low-pressure
inlet, flow passage and a casing. The main geometric parameters of the
tested regenerative pump are presented in Table 1 and also in Fig. 3.
The radial blades impeller for regenerative pump is shown in Fig. 4.

3. Experiments apparatus

The arrangement of test rig is indicated in Fig. 5. As it can be
seen a reservoir tank with the capacity of 150 liters was employed to
store and ultimately receive fluid. For flow rate adjustment, a control
valve was installed in the return line to the reservoir tank. The flow
rate was measured using a rotameter which was calibrated.

The pump was driven by a 500-watt induction motor operating at a
constant speed of 2900 rpm. For power consumption of the electro pump, a
watt meter with the accuracy of about [+ or -]1.5% was employed. Fig. 6
represents the performance curve of the electromotor. It was obtained
using a dynamometer, separately. So the power on the shaft can be
obtained to use in the calculation of pump hydraulic efficiency by Eq.
(1).

[mathematical expression not reproducible] (1)

where [P.sub.1] is power consumption of electromotor and [P.sub.2]
is Power on the shaft.

A proximity sensor was used to measure the rotational speed. The
inlet and outlet pressures were measured using two analog pressure
gauges.

Viscosity measurement of working fluids was carried out with a
Brookfield viscometer. The schematic of viscometer has been shown in
Fig. 7. The principle of operation of this kind of viscometer is to
rotate a spindle immersed in the fluid and measuring the viscous drag of
the test fluid against the spindle through the deflection of a
calibrated spring. The deflection of spring is diagnosed with a rotary
transducer. The viscometer has full scale range accuracy within [+ or
-]1%.

The following equations (equations 2-5) are used for drawing curves
of shear stress and viscosity of fluid versus shear rate.

[mathematical expression not reproducible] (2)

[mathematical expression not reproducible] (3)

[mathematical expression not reproducible] (4)

[mathematical expression not reproducible] (5)

where [r.sub.i] is shear stress, [D.sub.i] is diameter of spindle,
l is effective length of spindle and [N.sub.sp] is the rotational speed
of spindle.

Polyacrylamide (PAM) which is a soluble polymer in water was
employed to increase the viscosity of water, see Fig. 8. The technical
specifications for Polyacrylamide (grade A500) are presented in table 2.
Solutions prepared from dry powder Polyacrylamide (PAM) at two
concentrations of 1600 ppm and 2400 ppm. According to experimental
studies of Bjorneberg, the viscosity of PAM solution reduces by pumping.
The viscosity reduction is thought to result from breaking or shearing
the PAM molecules. He reported that pumping a 2400 ppm PAM solution just
once through a centrifugal pump reduces viscosity 15 to 20%; pumping
five times reduces viscosity approximately 50%. But there was not
significant change in viscosity of PAM solution through pumping more
than five times [18]. So it shows that to have a homogeneous solution it
is necessary to pump PAM solutions several times.

4. Testing procedures

As mentioned, the main objective of this experimental work is to
investigate the effect of viscosity change on the performance of
regenerative pumps. Thus, as summarized in Table 3, three fluids with
different viscosities were employed. In this study the working fluid was
pumped more than 30 times to have a homogeneous solution in each case.
Then, a sample was taken for measuring viscosity by Brookfield
viscometer as seen in Fig. 9.

For each working fluid, the pressures and power consumption were
measured with controlling the flow rate by the flow control valve. The
operating conditions are presented in Table 4 and the test rig
arrangement is shown in Fig. 10.

5. Results and discussions

Regenerative flow pumps confirm affinity laws as centrifugal and
axial pumps. So the experimental measurements of the regenerative pump
performance characteristics are expressed using dimensionless
parameters. The flow coefficient ([phi]), head coefficient ([psi]),
power coefficient (r) and hydraulic Efficiency ([eta]) which is defined
as the ratio between the hydraulic power transferred to the working
fluid and the mechanical power introduced into the system by the
impeller [13]. Pump characteristic flow, head, power and efficiency
coefficients can be expressed as follow:

[mathematical expression not reproducible] (6)

[mathematical expression not reproducible] (7)

[mathematical expression not reproducible] (8)

[mathematical expression not reproducible] (9)

where [U.sub.g] is tangential velocity, [A.sub.0] is the cross
section area of channel and [rho] is the density of working fluid.

Fig. 11 shows the characteristic curves of the regenerative pump,
using water as working fluid. As it can be seen the maximum hydraulic
efficiency occurs at [[phi].sub.n] = 0.47 with about 42.5%. At shut-off
condition ([phi] = 0), the head coefficient ([psi]) is about 5.3 and the
power coefficient ([tau]) is obtained about 4.4.

Fig. 12 indicates the characteristic curves of the regenerative
pump at three different viscosities of working fluid. The primary fluid
was water with the viscosity of 1 centipoise. As mentioned, a soluble
polymer (Polyacrylamide) in water was used to change the viscosity of
fluid. Solutions prepared from dry powder PAM at two concentrations of
1600 ppm and 2400 ppm. To have a homogeneous fluid, the solutions were
pumped more than 30 times. A Brookfield viscometer was used for
measuring the viscosities of solutions at these two concentrations.
Viscosities for these two solutions were 180 and 300 centipoises,
respectively.

As it can be seen from Fig. 12, a, by increasing the viscosity of
working fluid the head coefficient reduces. It is observed that for
higher flow coefficients (more than 0.5), there were not significant
changes in head coefficients. But as flow coefficient reduces, the
decline in head coefficient gradually increases. The percent of
reduction in head coefficient at shut-off point ([phi] = 0), for the
pump working with solution 1 (180 centipoise) and solution 2 (300
centipoise), relative to that of working with water, is 43.4% and 67%,
respectively.

Fig. 12, b shows the variations of power coefficient in terms of
flow coefficient. Trends of these curves are similar to head
coefficient. Viscosity enhancement reduces the power coefficient. The
reduction in power is significant at lower flow rates. The amount of
power coefficient at shut-off point is reduced about 50% by changing
viscosity from 1 to 180 cp. The value of power coefficient at shut-off
condition for regenerative pump working with solution 2 (300 centipoise)
is about 1.8 which is 59% lower than the power coefficient of the pump
working with water.

As summarized in Table 3, the impeller tip speed Reynolds number
([R.sub.T]) represented by equation 10 reduces by increasing the
viscosity of fluid. Thus, with reduction in Reynolds number, the
regenerative pump characteristics deteriorate.

[mathematical expression not reproducible] (10)

where [nu] represents kinematic viscosity.

The comparison of hydraulic efficiency curves for these three cases
is indicated in Fig. 12, c. It was not observed any significant changes
in efficiency curves at low flow coefficients. It because the more the
head coefficient reduces the more the power coefficient decreases. But
around design condition or in other words, around the best efficiency
point, the hydraulic efficiency of the pump reduces by increasing
viscosity of working fluid. At best efficiency point ([[phi].sub.n] =
0.47), the efficiency of the pump working with solution 1 (180 cp) is
about 40.2% which is 2.3% lower than the efficiency of the pump uses
water as working fluid. The efficiency of the pump working with solution
2 (300 cp) is about 33.7% which is 8.8% lower than the pump working with
water (1 cp). Eventually, it was found that increasing the viscosity of
working fluid would reduce the performance characteristics of the
regenerative pump. So, it seems that regenerative pumps are not good
choices for pumping fluids with very high viscosities.

6. Uncertainty analysis

Considering the output quantity denoted by R, is related to n
variables as:

R = R([x.sub.1],[x.sub.2],...,[x.sub.n]). (11)

The overall uncertainty, [W.sub.R] will obtain from relation (12):

[mathematical expression not reproducible] (12)

where [w.sub.n] is the associated uncertainty of independent
measurement [x.sub.n].

From equation 13, [delta]P,Q and [delta]Z are the independent
variables for calculating the pump head.

[mathematical expression not reproducible] (13)

The uncertainty in the measurement of flow rate was about [+ or
-]2%. And the pressures were measured with the uncertainty of [+ or
-]1.5%. The uncertainty for [delta]Z is about [+ or -]0.1%. The results
of uncertainty analysis are tabulated in Table 5. The average of
uncertainties is about 3% which indicates the acceptability of test
results.

7. Conclusions

In this study, the influence of fluid viscosity on the performance
of regenerative pump was investigated experimentally. The primary fluid
was water with viscosity of 1 centipoise. A soluble polymer in water
named polyacrylamide (PAM) was used to change the viscosity of fluid.
The working fluids were pumped more than 30 times to have homogeneous
solutions in each case. A Brookfield viscometer was employed for
measuring viscosity. Solutions prepared from dry powder PAM at two
concentrations of 1600 ppm and 2400 ppm. The measured viscosties for
these two solutions were 180 centipoises (solution 1) and 300
centipoises (solution 2) respectively. Results showed that increase in
the viscosity of working fluid would decrease the head coefficient. It
is observed that as flow coefficient reduces, the decline in head
coefficient gradually increases. It was found that viscosity enhancement
reduces the power coefficient. Similar to head coefficient, the
reduction in power is considerable at lower flow rates. According to the
comparison of the hydraulic efficiency curves, there was not any
significant change in efficiency curves at low flow coefficients. It
because the more the head coefficient reduces the more the power
coefficient decreases due to enhancement in viscosity of working fluid.
Contrary to low flow coefficients, around design flow coefficient and
high flow coefficients, the hydraulic efficiency of the pump reduces by
increasing viscosity of working fluid. The results of this experimental
study showed that viscosity enhancement would reduce the performance
characteristics of regenerative pumps. Thus, it could be concluded that
regenerative pumps are not suitable for pumping fluids with very high
viscosities or in other words regenerative pumps are not good choice in
low Reynolds numbers.

[16.] Maity, A.; Chandrashekharan, V. and Afzal, M.W. 2015.
Experimental and Numerical Investigation of Regenerative Centrifugal
Pump using CFD for Performance Enhancement, International Journal of
Current Engineering and Technology 5(4): 2898-2903. Available at
http://inpressco.com/category/ijcet.